Additive manufacturing, sometimes also referred to as 3-D printing, is a technique in which an article is formed using a 3-D printing apparatus that builds up the article in three dimensions in a number of printing steps that are usually controlled by a 3-D computer model, e.g. 3-D CAD data, of the article to be manufactured. For example, a sliced 3-D model of the article may be provided in which each slice is recreated by the printing apparatus in a discrete printing step. To this end, the printing apparatus may deposit a plurality of layers of one or more printing materials, which layers may be cured or otherwise hardened after deposition, e.g. using a laser to induce the curing process. An example of such a printing apparatus is disclosed in US 2010/0327479 A1.
Additive manufacturing techniques are rapidly gaining popularity because of their ability to perform rapid prototyping without the need for assembly or molding techniques to form the desired article. However, a fundamental problem associated with such techniques is that the strength of the accordingly manufactured article is often inferior to that of a corresponding article formed using traditional assembly methods. This is because the adhesion between adjoining materials, e.g. layers, fibers and/or grains, can be poor.
For instance, in additive manufacturing techniques based on powder sintering, the printed objects are composed of sintered grains where the neck between grains often exhibits low fracture strength. In additive manufacturing techniques where the grains or fibers are embedded in a matrix such as in ink jet or fiber printers, the adherence of the grains or the fibers to the matrix material might be weak. In the case of Fused Deposition Modelling (FDM) or dispensing techniques, layers of material are deposited on top of each other to produce a structure. It is often the case that at the interface of the layers, in particular when the underlying layer is cold for FDM or not compatible with the layer on top, the adhesion between the layers is weak. Consequently, articles with inferior mechanical properties are obtained. Furthermore, in case of multi-material prints the different printed materials can exhibit poor adhesion. This results in further weakened mechanical properties or even in the impossibility to produce the desired articles.
US 2014/0134335 A1 discloses a process for the production of coated filaments for use in an extrusion 3-D printing process in which filaments are coated with an additive prior to printing to improve inter-layer adhesion. This however alters the composition of the printing material, here filaments, which may be undesirable. It would be desirable to be able to improve such inter-layer adhesion using standard printing materials.